There is a growing need for semiconductor memory devices that are electrically erasable and programmable and do not require a refresh operation for data retention. Increase in storage capacity of a semiconductor memory device is also desirable. Flash memory devices provide a large storage capacity without a refresh operation and retain data even when power supply is interrupted. Therefore, flash memory devices are widely used in electronic devices, for example, portable electronic devices, that may undergo sudden power interruption.
A flash memory device, also known as an Electrically Erasable Programmable Read Only Memory (EEPROM), includes a memory cell array including floating gate transistors. The memory cell array includes a plurality of memory blocks. A plurality of bit lines are arranged in parallel in the memory blocks. Each of the memory blocks includes a plurality of strings (or NAND strings).
Each of the strings includes a string select transistor (SST), a ground select transistor (GST), and a plurality of floating gate transistors that are connected in series between the SST and the GST. Each of the floating gate transistors shares a source-drain terminal with the adjacent floating gate transistor.
Furthermore, a plurality of word lines are arranged across each of the strings. The control gates of the floating gate transistors are connected in common to each of the word lines.
In order to program memory cells including the floating gate transistors, the memory cells are first erased to have a predetermined threshold voltage (e.g., −3V). Thereafter, a high voltage (e.g., 20V) is applied to a word line, connected to a selected memory cell, to program the selected memory cell. For an accurate program operation, the threshold voltage of the selected memory cell must be increased while the threshold voltages of the unselected memory cells must be maintained without change.
However, when a program voltage is applied to a selected word line, the program voltage is applied not only to a selected memory cell, but also to an unselected memory cell connected to the selected word line. As a result, the unselected memory cell connected to the selected word line may become programmed. This accidental programming of the unselected memory cell connected to the selected word line is called a program disturbance. In particular, the program disturbance may be more problematic in a multi-level cell to which a high program voltage is applied.